This invention is related to a method for preparing phenolic aldehyde resin. More particularly, this invention relates to a method for preparing resorcinol-formaldehyde resin for use in an adhesive system for the adhesion of glass fiber to rubber to produce reinforced rubber goods.
Filamentary materials have been used extensively as reinforcing material in rubber to produce reinforced rubber products, such as pneumatic tires, power-drive belts, conveyor belts, high pressure hoses and the like. The filamentary materials that are used to reinforce rubber material include naturally occurring or synthetic filaments and may be in the form of individual fibers, groups of fibers in the form of strand, rope, cord, roving fabric and the like. The naturally occurring fibers include cotton, silk, ramie and the synthetic fibers include rayon, nylon, polyester and glass fibers.
Glass fibers are excellent filamentary material for reinforced rubber and are superior to the natural and synthetic organic filamentary materials, since the glass fibers do not become elongated or deformed under stress to the extent of the other filamentary materials. Unlike other filamentary materials, particular combinations of glass fibers with encapsulating coating cooperate to yield reinforced rubber materials that have greater strength than even the glass or the coating material alone. While filamentary materials, other than glass fibers, which are subject to substantial stress elongation, are essentially limited in tensile strength to the basic strength of their fibers, even if coated properly, coated glass fibers have greater strength than the glass alone. For example, the low modulus of elasticity of glass may be exploited to provide reinforced rubber tires having superior road performance, if an appropriate coating medium is provided to transfer stresses to all fibers in the glass fiber cord so that loading throughout is substantially uniform. This phenomenon is illustrated by the observation that a typical uncoated fiber glass cord (G75,5/0, filament count 2,000 i.e., 2,000 filaments of G fiber of about 9.14 micro meter diameter, 15,120 meters per kilogram, 5 strands per cord) has a tensile strength of about 35 to 40 pounds (156 to 178 newtons) ASTM test G178-52, but when coated with a coating, e.g., resorcinol formaldehyde latex coating, such a cord has a tensile strength of about 50 to about 70 pounds (220 to 311 newtons).
The above-mentioned coated glass fiber cord, GT-75,5/0 has found particular utility in the reinforcement of rubber for use in power transmission belts and fiber glass reinforced tires and the like. In such a coated glass fiber cord, a resorcinol formaldehyde latex coating is used as the adhesive system to transfer the stresses and to provide adhesion between the glass fibers and the rubber. Typically, the resorcinol formaldehyde, or resorcinol-phenol-aldehyde resin, useful in adhesive systems for the adhesion of glass fibers to rubber is produced by a method using a basic pH environment, i.e., around a pH of 8 to 10. The phenolic aldehyde resin usually has an aldehyde level to phenolic compound level, usually resorcinol, of 0.4 to 0.8 to one phenolic compound on a mole basis. Such a resin is characterized by a low degree of polymerization and minimum molecular weight. A particularly useful phenol aldehyde condensate, which is a resorcinol formaldehyde resin, has a ratio of 0.6 formaldehyde to 1 resorcinol, and is sold under the designation Penacolite.RTM.R-2200 resin.
There are several methods known in the art for preparing phenol aldehyde polymers to be used in adhesive systems. As early as 1947 in U.S. Pat. No. 2,385,372 (Rhodes) a permanently fusible resin was prepared from dihydroxybenzene (resorcinol) and an aldehyde in two stages so that a catalyst was not present during the stages of a reaction. It was theorized that having the catalyst present in the early stages of the reaction would cause a resin to be too thick for the removal of the water produced by the reaction. The problem was overcome by employing a two-stage reaction, wherein the dihydroxybenzene (resorcinol) is reacted at reflux conditions with the aldehyde without a catalyst until a major portion of the reaction is completed. Then either an alkaline or acid catalyst is added and the last increment of aldehyde reacts with the dihydroxybenzene.
Also a several stage reaction has been employed to produce a phenolic aldehyde condensate in U.S. Pat. No. 4,025,454 (Rouzier), wherein a pre-condensate of formaldehyde, resorcinol and a para-substituted phenol and a pre-condensate of resorcinol and formaldehyde are used. In a first stage, resorcinol and a para-substituted phenol with two active methylene groups are condensed in the presence of an acid catalyst. In the second stage formaldehyde is condensed with the product of the first stage in an alkaline medium. Then in the third stage the product of the second stage is dissolved in water along with a resorcinol pre-condensate to form the phenoplastic system, which is a mixture of phenol aldehyde condensates, then combined with an elastomeric latex to form the adhesive for textile fibers.
Also, in U.S. Pat. No. 3,956,205 (Higginbottom) a resole is produced by a two-stage reaction. The first stage of the reaction is carried out under novolac forming conditions, where an acid catalyst is used to give a pK that is less than 5. In the first stage, one mole of phenol is reacted with 0.05 to 0.30 moles of formaldehyde in order to favor the formation of the dimer polymer and suppress the formation of higher oligomers. In the second stage the reaction is conducted in the presence of a basic catalyst, which has a pK greater than 9 with the addition of 1.75 to 3.5 moles of formaldehyde per mole of original phenol for the resole reaction. At the end of the reaction, the catalyst is neutralized by addition of acid to reduce the pH to between 6 and 8.5 to produce the resole resin.
The phenolic aldehyde resins of the prior art, such as Penacolite.RTM. resorcinol formaldehyde resin, and those produced by the aforementioned multi-stage processes can be improved upon for use in an adhesive system to coat glass fibers that are used to reinforce rubber products. An improvement of the phenolic aldehyde resin is desired to give the coated glass fibers more flexibility and better resistance to compression fatigue breakage, thereby yielding more durable and longer lasting reinforced rubber products.
One of the many reinforced rubber products that would benefit from the use of coated glass fibers having more flexibility and better resistance to compression fatigue are pneumatic tires. A bias belted tire having coated glass fibers that have more flexibility and better resistance to compression fatigue would have improved wear characteristics and would give extended mileage. Also radial tires having glass fiber belts, alone or in combination with other filamentary material belts, containing coated glass fibers having more flexibility and better resistance to compression fatigue would give extended mileage and improved handling.
It is an object of the present invention to provide a method for preparing a thermoplastic, phenolic formaldehyde resin having improved flexibility and having toughness and which is comprised of a substantial amount of the trimer polymer and to provide the said resin composition for use in an adhesive system used in coating glass fibers to render the glass fibers more flexible and more resistant to compression fatigue breakage.